Many electronic devices include one or more rechargeable batteries that require external power to recharge from time to time. Often, these devices may be charged using a similar power cord or connector, for example a universal serial bus (“USB”) connector. However, despite having common connection types, devices often require separate power supplies with different power outputs. These multiple power supplies can be burdensome to use, store, and transport from place to place. As a result, the benefits of device portability may be substantially limited.
Furthermore, charging cords may be unsafe to use in certain circumstances. For example, a driver of a vehicle may become distracted attempting to plug an electronic device into a vehicle charger. In another example, a charging cord may present a tripping hazard if left unattended.
To account for these and other shortcomings of portable electronic devices, some devices include an inductive charging device. The user may simply place the electronic device on an inductive charging surface of a charging device in order to transfer energy from the charging device to the electronic device. The charging device transfers energy to the electronic device through inductively coupling between a transmitter coil in the charging device and a receiver coil in the electronic device. But as the size of many electronic devices continues to decrease, the transmitter coil may have an inductance that is lower than desired. This lower inductance can lead to large circulating currents in the inductive charging device in the transmitter device, which can result in large power losses. In situations where a high amount of power is needed by the receiver device, the large power losses may be acceptable. But when only a small amount of power is needed on the receiver side, the losses in the transmitter coil may be unacceptable and can cause thermal problems or unacceptably large parasitic standby power, also known as vampire power.